Enantioselective catalysis using chiral metal complexes provides one of the most general and flexible methods for achieving asymmetric organic reactions. Metallic elements possess a variety of catalytic activities, and permutations of organic ligands or other auxiliary groups directing the steric course of the reaction are practically unlimited. Efficient ligands must be endowed with, for example, suitable functionality, an appropriate element of symmetry, substituents capable of differentiating space either electronically or sterically, and skeletal rigidity or flexibility.
Among the asymmetric organic reactions catalyzed by chiral transition metal complexes, asymmetric hydrogenation has been one of the best studied, due in large part to the fact that it is the basis for the first commercialized catalytic asymmetric process. See, for example, ApSimon, et al., Tetrahedron, 1986, 42, 5157.
Some of the more interesting of the asymmetric hydrogenation catalysts are those derived from BINAP [2,2'-bis(diphenylphosphino)-1,1'-binaphthyl]. See, for example, U.S. Pat. Nos. 4,691,037; 4,739,084; 4,739,085; and 4,766,227. Unlike the more classical models of chiral (asymmetric) molecules, chirality in the case of the BINAP compounds arises from the restricted rotation about the single bond joining the naphthalene rings. Because of such restricted rotation, perpendicular disymmetric planes result. Isomers arising from this type of asymmetry are termed atropisomers.
Cationic rhodium-BINAP complexes have been shown to catalyze the isomerization of allylamines to chiral enamines in 94-96% ee. Also, hydrogenations of geraniol and nerol (bis-unsaturated alcohols) using rhodium-BINAP complexes produce products in about 50% ee's. The synthesis of BINAP derivatives bearing groups other than phenyl on phosphorus such as paramethylphenyl and cyclohexyl have also been prepared. Inoue, et al., Chem. Lett., 1985, 1007. Other rhodium complexes have been prepared. See, for example, Bosnich, et al., J. Am. Chem. Soc., 103, 2273 (1981).
The BINAP ruthenium complexes have been used to catalyze a variety of asymmetric hydrogenations including the reduction of enamides, alkyl and aryl-substituted acrylic acids, homoallylic alcohols and functionalized ketones. See Noyori, et al., Modern Synthetic Methods, 1989, 5, 115, incorporated herein by reference. While these complexes are effective in facilitating the asymmetric reduction of the above compounds, they are difficult to prepare and expensive to produce.